1. Field of the Invention
The present invention relates to a printing method and apparatus for a printer, and more particularly, to a printing method and apparatus for a thermal transfer printer, or the like, which prints without raising a printing and reduces the size of top and bottom-margins of a sheet of paper when printing color images.
2. Description of the Related Art
Generally speaking, a thermal transfer printer has a thermal printing head with a heating element, and a film-shaped ribbon coated with ink. The ribbon is placed between paper and the thermal printing head and ink in the ribbon is sublimed in a predetermined pattern onto a sheet of paper through heat and pressure. In order to obtain full color printing, a color thermal transfer printer performs printing three times in color-separated yellow, magenta and cyan, respectively.
FIG. 1 illustrates a conventional color ink ribbon employed in the conventional printing method and apparatus for a thermal transfer printer. FIG. 2 illustrates a conventional printer using the ribbon of FIG. 1. The printer of FIG. 2 is a platen type in which a sheet of paper is delivered in close contact with a drum and guide rollers. Referring to FIG. 2, a plurality of guide rollers 5, 5', 6 and 6' are in contact with the circumference of a drum 4. A paper feeding cassette 9 is provided at a predetermined position under drum 4. Paper 3 is supplied to drum 4 by a paper feeding cam 7.
Ink ribbon 1 of FIG. 1 on which yellow (Y), magenta (M), and cyan (C) portions are provided serially as stripes extending across the width of the ink ribbons, is placed between printing head 2 and paper 3 and is wound in one direction by a driving means (not shown). Printing head 2 is movable up and down as shown in FIG. 2, to press or release ink ribbon 1.
According to the printing method of the conventional thermal transfer printer constructed as above, paper 3 is fed to drum 4 by paper feeding cam 7. Paper 3 having reached the drum is conveyed in close contact with guide rollers 5 and 5' and drum 4. When the leading edge of conveyed paper 3 reaches guide roller 6 located beyond an initial printing position, drum 4 stops.
Then ink ribbon 1 is transported according to a printing signal. One of the three portions (Y, M and C), the leading edge of portion Y is placed to coincide with the initial printing position of paper 3. Subsequently, printing head 2 is lowered to thermally press (i.e., press and heat) ink ribbon 1 and paper 3, and simultaneously drum 4 rotates one revolution. This is the printing process for transferring a yellow image onto paper 3.
Next, printing head 2 is lifted, and the leading edge of portion M of ink ribbon 1 is set to coincide with the initial printing position of paper 3. Printing head 2 is lowered again and ink ribbon 1 and paper 3 are thermally pressed and drum 4 rotates one revolution, thereby printing a magenta image onto the paper to be superimposed on the yellow image. The printing of the third color, cyan, is carried out in the same manner as above.
Such a conventional printing method, however, has a problem in that the printing time is undesirably lengthened since printing head 2 must be raised whenever ink ribbon 1 is transported to position a specific color portion in opposition to print head 2.
Further, in the conventional printing method, since printing is initialized when the leading edge of paper 3 advances to guide roller 6 as shown in FIG. 2, to prevent paper 3 from separating from drum 4 due to the compression force of printing head 2 and the rotation of drum 4 during printing, the top margin of paper 3 is undesirably maintained as an interval a between printing head 2 and guide roller 6, with the bottom margin of paper 3 kept as a symmetrical interval a'.